A plasma display panel (hereinafter, called a “PDP”) is a flat display apparatus that makes use of radiation from gas discharges. PDPs can easily perform high-speed display and be large in size, and are widely used in fields such as video display apparatuses and public information display apparatuses. There are two types of PDPs, namely the direct current type (DC type) and alternating current type (AC type). Surface discharge AC type PDPs have been commercialized due to having a great amount of technological potential in terms of lifetime and increases in size. FIG. 8 is a schematic view showing the structure of a discharge cell that is a discharge unit in a general AC type PDP. A PDP 1x shown in FIG. 8 includes a front panel 2 and a back panel 9 that have been disposed in opposition to each other. In the front panel 2, a plurality of display electrode pairs 6, each including a scan electrode 5 and a sustain electrode 4, have been arranged on one face of a front panel glass 3, and a dielectric layer 7 and a surface layer 8 have been formed thereon in the stated order so as to cover the display electrode pairs 6. The scan electrodes 5 and sustain electrodes 4 are constituted respectively from transparent electrodes 51 and 41 and bus lines 52 and 42 formed thereon.
The dielectric layer 7 is formed from low melting point glass whose softening point is in the range of 550° C. to 600° C., and has a current limiting function that is unique to AC type PDPs.
The surface layer 8 protects the dielectric layer 7 and display electrode pairs 6 from the bombardment of ions generated by plasma discharges, as well as efficiently emits secondary electrons, thereby reducing the firing voltage. In general, the surface layer 8 is formed by using a vacuum deposition method or printing method to form a layer of magnesium oxide (MgO), which is superior in terms of secondary electron emission, sputter resistance, and transparency. There are cases in which a structure similar to the surface layer 8 is provided as a protective layer for the purpose of protecting the dielectric layer 7 and display electrode pairs 6, as well as ensuring secondary electron emission.
In the back panel 9, a plurality of data (address) electrodes 11 for writing image data have been provided on a back panel glass 10 so as to orthogonally intersect the display electrode pairs 6 on the front panel 2. A dielectric layer 12 formed from low melting point glass has been provided on the back panel glass 10 so as to cover the data electrodes 11. Ribs 13 made of low melting point glass have been formed to a predetermined height on the dielectric layer 12 at borders between adjacent discharge cells (not depicted). The ribs 13 include pattern portions 1231 and 1232 that combine to form a lattice pattern or the like, so as to demarcate discharge spaces 15. Phosphor layers 14 (phosphor layers 14R, 14G, and 14B) have been formed on the surface of the dielectric layer 12 and side walls of the ribs 13 by the application and baking of red, green, and blue phosphor inks and baking thereof.
The front panel 2 and the back panel 9 are disposed so that the display electrode pairs 6 and the data electrodes 11 intersect each other via the discharge spaces 15, after which a periphery of the front panel 2 and back panel 9 is sealed. At this time, a rare gas mixture including Xe and Ne, Xe and He, or the like is enclosed as a discharge gas at a pressure of several tens of kPa in the discharge spaces 15 sealed between the front panel 2 and back panel 9. This completes the formation of the PDP 1x. 
In PDPs, image display is performed with the use of a gray-scale expression system (e.g., an intra-field time-division display system) that divides one image field into a plurality of sub fields (S.F.). However, in recent years there has been desire for low-power driving in electrical appliances, and the same desire exists for PDPs as well. Since discharge cells are made smaller and increased in number in high definition PDPs, there is the problem of requiring a higher operating voltage in order to increase the reliability of writing discharges. The operating voltage of a PDP depends on the secondary electron emission coefficient (γ) of the surface layer. Here, the value of γ depends on the material of the surface layer and the discharge gas, and γ is known to increase as the work function of the material decreases. In view of this, patent document 4 discloses the use of calcium oxide (CaO), strontium oxide (SrO), barium oxide (BaO) or the like as the main component of the protective layer. Doing so enables the formation of a high γ film that has a more favorable secondary electron emission property than MgO, and enables the PDP to be driven at a relatively low voltage.    Patent document 1: Japanese Patent Application Publication No. H08-236028    Patent document 2: Japanese Patent Application Publication No. H10-334809    Patent document 3: Japanese Patent Application Publication No. 2006-54158    Patent document 4: Japanese Patent Application Publication No. 2002-231129    Patent document 5: WO 2005/043578